1. Field of the Invention
Our invention relates generally to the field of advanced signal processing employing correlation of electrical signals, and specifically to an acousto-optic system for the real-time correlation of very wideband signals.
2. Background of the Invention
The correlation of electrical signals has a fundamental role to play in many signal processing applications. Correlation generally has its roots in the employment of an electrical filter which, when matched to the particular electrical signal to be processed, provides a maximum signal to noise gain. In some instances two signals to be analyzed are cross-correlated, where the processing can be viewed as determining the degree of similarity between the two signals. Electrical correlation can be effected by means of delay lines, such as surface acoustic wave compressors, or by digital means. Delay lines typically limit the achievable processing time-bandwidth product to on the order of 1000. Digital techniques are typically bandwidth-limited due to the need to sample and digitize the waveform to be analyzed at a data rate equal to or greater than twice the highest frequency in the signal. For many applications, both large time-bandwidth product and very wide bandwidths are required. A standard technique, known as stretch processing, exists for obtaining the correlation of large time-bandwidth product, very wideband linear frequency modulated signals, but is not applicable to other than linear frequency modulated signals.
An alternate approach, and one particularly suited to the correlation of high time-bandwidth product, very wideband signals, is available through the employment of acousto-optic delay line devices. Acousto-optic correlator architectures have been disclosed in U.S. Pat. Nos. 3,634,749; 4,225,938; 4,326,778; 4,421,388;and 4,558,925. As a preface to the discussion of these references with respect to the present invention, the invention may be summarized as follows.
This invention discloses an acousto-optic architecture for obtaining the complex correlation between two signals. The processor has been specifically configured to accommodate very wideband signals typical of advanced radar and communication intercept processing as well as commercial applications such as seismic processing, computerized tomography, ultrasonic imaging, and nuclear magnetic resonance. This correlator is general purpose in the sense that completely arbitrary waveform modulations can be accommodated, and the desired correlation magnitude and phase can be obtained. An interferometric time-integrating correlation algorithm is employed to achieve the complex correlation output on a linear optical detector array. As one feature, complex correlation is achieved by modulating the correlation onto a spatial carrier whose frequency can be conveniently selected. Subsequent to detection, an electronic bandpass filter is employed to remove the inherent undesirable low-frequency bias terms, and coherent in-phase and quadrature detection is performed and the calculation of the correlation magnitude and phase is achieved.
Now turning to the prior art, an early description of a time-integrating acousto-optic correlator is disclosed in U.S. Pat. No. 3,634,749, entitled "Acousto-Optical Signal Processing System". In this patent, two counterpropagating acoustic waves diffract an incoming optical beam, which is then imaged onto a linear detector array. Spatial filtering to remove the undiffracted light is performed within an imaging lens system. The correlation that results after time integration is intentionally modulated by a spatial carrier which allows the correlation to be separated from undesirable low-frequency bias terms by means of an electronic bandpass filter. A significant difference from the instant invention is that the spatial carrier frequency is set by the center frequency of the Bragg cell and thus cannot be modified. The resultant correlation is then removed from the carrier by conventional electronic demodulation techniques.
An enhanced version of this early invention appears in U.S. Pat. No. 4,326,778, entitled "Acousto-Optic Time Integrating Correlator". The application of this processor was to the correlation of radio-frequency signals to perform time-difference-of-arrival intercept of spread-spectrum emitters. In this application, the correlation is once again modulated onto a spatial carrier at the output of the linear optical detector array. In this case the spatial carrier frequency is a function of the difference between the Bragg cell center frequency and the center frequency of the intercepted signal, and is thus not known a priori as it is in our application. The undesirable bias terms can be removed in this case by performing an additional correlation, where one of the signals is phase shifted 180.degree., and subtracting this result from the first correlation, which is a significantly different approach than that of the instant invention.
Another correlator is disclosed in U.S. Pat. No. 4,421,388, entitled "Acousto-Optic Time Integrating Frequency Scanning Correlator", and is an acousto-optic two-dimensional frequency scanning correlator for cross correlating signals which are separated in frequency. Two coherent light beams, which are derived from the same laser, are projected into respective Bragg cells which contain the signals to be cross-correlated. The respective output beams are compressed in the x-direction and expanded in the y-direction and are made incident on an acousto-optical correlator device having chirp signals counterpropagating thereacross. The optical output is imaged onto a time-integrating photodiode array which provides the desired cross-correlation as a function of the frequency offset. This correlator is especially useful in radar applications where the target return may be significantly Doppler shifted from the reference signal input into the correlator.
Of the references, U.S. Pat. No. 4,558,925, entitled "Multi-Function Acousto-Optic Signal Processor", appears to be the most similar in optical configuration to the correlator of the present invention in that it employs separate Bragg cells in two separate legs of a Mach-Zehnder interferometer. Each of the Bragg cells are illuminated with coherent light derived from the same laser. The diffracted and undiffracted beams from each of the Bragg cells are then combined in a beamsplitter and subsequently Schlieren-imaged back onto a linear optical detector array. The square-law detection process then yields the correlation result on a spatial carrier in addition to the inherent undesirable low-frequency bias terms. Due to the nature of the imaging system employed, wherein a single Schlieren imaging system is employed after combining the two beams, the bandwidth capability of the system is limited due to the physical limitations on lens f-numbers. However, a major difference from the instant invention is that the spatial carrier fringe frequency is a function of the difference between the Bragg cell center frequency and the center frequency of the intercepted signal, and is thus not known a priori. Because of this, a bandpass filter, such as suggested by U.S. Pat. 3,634,749, cannot be employed to remove the undesirable low-frequency bias terms. Finally, the system of this patent is configured for a signal intercept application, and no means are provided for the measurement of the correlation phase as is provided in the instant invention.